High pitch buzz in stage monitors

[Xypheres]

I was driving 6 different stage monitors for a band practice last night and somehow, there was a high pitch buzz. I used 3 different amps, a 650W/channel driven to 80%, a 750W/channel driven to 80% and a 1800W/channel driven to 60%. The stage monitors were all 15" One Systems, at a power rating of 800W.

My source to amps were run over a distance of about 10', from a A&H stage monitor board. There was a power cable, used to power the 1800W amp running next to the signal lines to the amps.

On stage were 6 Shure SLX wireless systems, 6 DIs, 2 for Acoustic, 2 for keyboards, 1 for violin, 1 for bass, 2 57s for electric guitar and full drum mic set.
The amps were all grounded and so were the DIs.

Anyone can tell me what might be the possible problem? Btw, the 3 amps are stacked together.

Thanks

 

[Dongara]

Firstly, was the buzz in all monitors or only in one or two? If it is in one or two then there may be an electrical grounding problem or wiring problem in that monitor.

If it is in all monitors then it is the output from the board to power amp area. Remove all the inputs and see if the buzzing stops. If it does, then return each input until the buzzing starts again, this will tell you which input is causing the problem. Replace the wiring for that input and try again. If the problem still persists then replace the source.

Faultfinding. It's the hard side of doing sound, but something that you will have to get used to.

Once you have a setup that works, keep a book on it so the next time something goes wrong, compare what is different and that will make the process easier.

cheers,

Mark.

 

[iansoh]

I know this is probably a dead topic, but just to share my little 2 cents from what i've learnt, and solutions that i've discovered by trial and error..

Grounding is not always the best thing. protects human lives maybe, but certainly not the audio signals. In fact, grounding more often than not creates problems. If you're experiencing a high frequency, consistent buzz that does not change with signal level but changes with amp levels, then u probably have a ground loop buzz. A ground loop buzz is caused by a small constant current flowing from the ground of one device to the ground of another device and affecting the audio signals in there. This happens when devices by different manufacturers have different electrical potentials w.r.t ground.


Consider the following countermeasures, in the following order:

1) Ground lift all the DIs.

2) Use balanced connections from mixer outputs to amps. If you're using unbalanced 1/4" TS instrument cables for your amp connections, then you are exposing the audio signals to high risk. remember to use 1/4" TRS (mixer side) to 1/4" TRS or XLR (amp side).

3) Use balanced connections for all devices which are connected to the mixer via line input, i.e. the Shure SLX receivers. make sure it's connected via BALANCED OUT and not the UNBALANCED OUT. for unbalanced output devices, e.g. CD players or laptops, connect them via DI box (with ground lifted).

Usually by this step the buzz will all be eliminated. if still doesn't work, then:

4) Eliminate the use of grounded power cords. Try this as a last resort, and one by one only. Maybe you don't want to do this on devices with metal chassis. Exercise discretion and caution with this step as you are potentially endangering human lives.

 

[MadWerewolfBoy]

"If you're using unbalanced 1/4" TS instrument cables for your amp connections, then you are exposing the audio signals to high risk."

what does high risk here mean O_O

 

[iansoh]

Unbalanced signals are much more susceptible to interference from power lines, wall-wart power transformers and radio frequencies.